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What can be said about the quantity $$\gcd(N/q^{\alpha},\sigma(N/q^{\alpha}))$$ where $N$ is an odd perfect number and $q^{\alpha} \parallel N$? In particular, can one prove that it is always greater than one?

Note that, when $q = p$ is the special prime of $N$, then since we can express $N$ as $$N = p^k m^2$$ (where $p \equiv k \equiv 1 \pmod 4$ and $\gcd(p,m)=1$), we obtain $$\gcd(N/q^{\alpha},\sigma(N/q^{\alpha})) = \gcd(m^2,\sigma(m^2)) = \sigma(m^2)/p^k = \sigma(N/q^{\alpha})/q^{\alpha},$$ which is always greater than one.

The situation is much more complicated when $q \neq p$.

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  • $\begingroup$ You have $\sigma(n/p^\alpha)=\frac{2n}{p^{\alpha}+p^{\alpha-1}+\cdots+1}$, so the gcd is $\frac{n}{p^\alpha (p^\alpha + \cdots + 1)}$, or twice that if $\alpha$ is odd $\endgroup$ Commented Nov 5, 2023 at 4:03
  • $\begingroup$ Thank you for your time and attention, @CommandMaster. I think that should be $$\sigma(n/p^{\alpha})=\frac{2n p^{\alpha} (p - 1)}{p^{\alpha + 1} - 1},$$ if I am not mistaken? $\endgroup$ Commented Nov 5, 2023 at 4:10
  • $\begingroup$ $n/{p^\alpha}$ and $p^\alpha$ are coprime, so $\sigma(n/{p^\alpha})\sigma(p^\alpha)=\sigma(n)=2n$, so $\sigma(n/{p^\alpha}) = \frac{2n}{\sigma(p^\alpha)} = \frac{2n}{p^\alpha + p^{\alpha-1} + \cdots + 1}$ $\endgroup$ Commented Nov 5, 2023 at 4:12
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    $\begingroup$ @CommandMaster: So are you saying that $$\gcd(N/q^{\alpha},\sigma(N/q^{\alpha}))={\begin{cases}{\frac{N}{q^{\alpha} \sigma(q^{\alpha})}, \alpha \equiv 0 \pmod 2 \\ \frac{2N}{q^{\alpha} \sigma(q^{\alpha})}, \alpha \equiv 1 \pmod 2}\end{cases}}$$ $\endgroup$ Commented Nov 5, 2023 at 4:16
  • $\begingroup$ Ahh yes, now I agree to your last comment. $\endgroup$ Commented Nov 5, 2023 at 4:19

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From the comments:

$$\gcd(N/q^\alpha,\sigma(N/q^\alpha)) = \begin{cases} \dfrac{N}{q^\alpha \sigma(q^\alpha)}, & \alpha \equiv 0 \pmod 2, \\[8pt] \dfrac{2N}{q^\alpha \sigma(q^\alpha)}, & \alpha \equiv 1 \pmod 2. \end{cases}$$

Since it is known that the index $$\sigma(N/q^{\alpha})/q^{\alpha} \geq 3$$ and since $$\sigma(q^{\alpha})\sigma(N/q^{\alpha})=\sigma(N)=2N,$$ because $N$ is perfect and $\gcd(q^{\alpha},N/q^{\alpha})=1$, then we obtain $$\sigma(N/q^{\alpha})/q^{\alpha} = \frac{2N}{q^{\alpha} \sigma(q^{\alpha})}.$$

It follows that

$$\gcd(N/q^{\alpha},\sigma(N/q^{\alpha})) \geq \dfrac{3}{2} > 1,$$

which proves the original claim.

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